Diaper "rash" problem *graphic pic*

A week ago my daughter woke up in the morning with a red mark on her butt and two smaller marks on her labia. The two smaller marks are almost entirely gone now but the mark on her butt looks raw. It's smaller than a dime, and doesn't seem to bother her. I've switched her into disposables so I could use a non cloth diaper sage cream. I'm pretty sure there was ammonia build up in the diaper that caused the issue. Anyone experience this? Advise? I'm stuck on if I should take her into see a pediatrician because it doesn't seem to bother her but I worry. Any thoughts would be appreciated.